In general, an extrusion machine comprises a barrel of elongate shape equipped with a bore extending along the entire length of the said barrel and in which there is a liner that envelops the screws.
When the extrusion machine comprises two parallel-axis screws, the liner comprises two parallel-axis cylindrical bores in which the screws are placed, and these screws may or may not intermesh.
The screws are fitted on their periphery with, in particular, helically-wound flights which carry in a downstream direction the substance which is introduced via an upstream end of the barrel.
The substance is thus carried along as far as the downstream end of the barrel, which may be fitted with an extrusion die or alternatively simply have an outlet orifice if the substance is not to be subjected to an extrusion operation.
In order to vary the treatment to be carried out as the substance progresses along the barrel, the screws generally consist of successive regions which have flights of different shape or pitch or which comprise specific treatment members.
Until now, the bore in the barrel was obtained by making two secant drillings which form two longitudinal ribs where they intersect and by machining these longitudinal ribs to obtain a flat surface between the two drillings so as to produce a bore formed of two half lobes joined by flat portions.
This technique therefore requires a number of machining tools, particularly a boring machine for the drillings and a milling machine to produce the flat surfaces.
Furthermore, it is necessary when performing the stage of grinding the bore of the barrel, to use special-purpose tools of special shape, given that this bore is formed of a series of curved portions and flat portions.
These various operations and the change of tooling for each of these operations significantly increases the cost of the barrel and the time taken to machine the bore in the barrel.